In general, a hydraulic construction machine, such as a hydraulic excavator, includes a diesel engine as a prime mover. At least one variable displacement hydraulic pump is driven by the engine, and a plurality of hydraulic actuators are driven by a hydraulic fluid delivered from the hydraulic pump, thereby performing necessary work. The diesel engine is provided with input means, such as a throttle dial, for commanding a target revolution speed. In accordance with the target revolution speed, the fuel injection amount is controlled and the revolution speed is also controlled. Further, the hydraulic pump is provided with pump absorption torque control means for horsepower control. The pump absorption torque control means executes control such that, when pump delivery pressure rises, pump tilting is reduced to avoid pump absorption torque from increasing over a preset value (maximum absorption torque).
Also, in a hydraulic construction machine, such as a hydraulic excavator, it is generally practiced to provide mode selection means separately from input means, such as a throttle dial, for commanding a target revolution speed, and to control the engine revolution speed by setting a control mode (work mode), such as an economy mode, through the mode selection means. In the economy mode, the engine revolution speed is reduced and therefore fuel economy is improved.
JP-A-62-160331 discloses a technique that the relationship between the revolution speed of a prime mover and the displacement of a hydraulic pump is preset in plural sets, a working state is determined using various detection means, and one of the plural sets is selected in accordance with the determination result and a signal from a mode selection switch to automatically switch over a control mode, whereby the revolution speed of the prime mover and the displacement of the hydraulic pump are controlled so as to make the maximum delivery rate of the hydraulic pump adapted for the working state.    Patent Document 1: JP-A-62-160331